Metal halide lamps that are made with curved or arched arc tubes must be operated horizontally with the curve of the arc tube in the upward or arch uppermost position (see U.S. Pat. No. 3,858,078). To accomplish this, the base and socket must provide a means of interlocking such that when the lamp is screwed into the socket firmly, the arc tube curve will be upward. Presently, the socket used has a notch and the base has a pin added to it so that when the lamp is screwed into the socket, it will stop in a predetermined position. This is in contrast to the usual screw type base that is used with prior art commercial metal halide lamps; a screw type base is not a positioning base and can be used in prior art lamps since the arc tubes thereof are straight cylinders.
Referring now to FIGS. 1 and 2, which will aid in clearly illustrating any problems associated with the prior art lamp bases. FIG. 1 illustrates a typical lamp base 10 comprised of a base shell 12, a base eyelet 14 and a base pin 16. FIG. 1 also illustrates a headed brass pin 16 which is affixed into and protrudes from base shell 12 in order to position the curved arc tube of a metal halide lamp. FIG. 2 illustrates a cross section of a typical socket 20 made of a porcelain casing 22 that has been partially cut away such that notch 24 can be seen. To make the pin and notch arrangement workable, a given dimension of X, X being about one inch with a positive tolerance of 1/32 of an inch was established between base eyelet 14 and pin 16, in FIG. 1. Notch 24 (illustrated in FIG. 2) in the socket shell 26 is located such that when base pin 16 is within the dimensions stated, eyelet 14 on base shell 12 will make contact with tab 28 in the socket. The tolerance of plus 1/32 of an inch translates into rotational tolerance of about 45 degrees of rotation or about 5/8 of an inch distance along the crest of the thread on which the pin is located.
The present method of pinning and attaching the base to the lamp is as follows: a hole is drilled in base shell 12 on the crest of the thread to meet the dimension stated with respect to FIG. 1. A headed brass pin 16 is then inserted into the hole with the head on the inside of base shell 12 and securely soldered in place. Since the soldering discolors the brass, the use of a nickel base is necessary for good cosmetics. During the lamp sealing process, four dimples are molded into the seal to permit a threaded inner shell having tabs with corresponding dimples to be snapped in place. Once in place, base shell 12 is securely screwed onto the inner shell and then is staked or pierced in three places to lock it to the inner shell.
When sealing lamps with a curved arc tube, the tip on the arc tube is located with respect to one of the dimples in the mold. In the basing process, the inner shell is selectively snapped into place such that the thread location on the inner shell corresponds to the dimple orientation accomplished during the sealing process. When the base is screwed onto the inner shell, the pin should be approximately 90 degrees counterclockwise from the tip on the curved portion of the arc tube when viewed from the dome end of the metal halide lamp. FIG. 3 illustrates a metal halide discharge lamp 30 when viewed from the top dome end looking down onto the lamp. Lamp 30 has an outer envelope 32, a curved arc tube 34, arc tube pressed end 36, an arc tube tip 38, a base shell 40 and a locating pin 42 similar to FIG. 1. FIG. 3 also illustrates a plane of the arc tube and support structure which runs along the line B-B'. A 90 degree angle is marked between the plane of the arc tube B-B' and the plane created by the arc tube and the locating pin marked C-C'.
The position orientation or locating arrangement in the past has not been very accurate, since the base has normally been manually screwed onto the inner shell until snug then the pin location is checked with respect to plane B-B' to see if the angle between them is 90 degrees .+-.5 degrees. If the angle is not within this range, the base must be loosened or tightened to meet the requirement. This action allows for a certain amount of movement to meet the angular requirement. however if the base is backed off too much from the snug position, the base will be loose after staking. If the base is over torqued, the lamp seal will crack. In either case, the lamp is rejected. The problems associated with the present pin locating methods are as follows: possible rejections may occur due to the loose base or cracked seal and over torquing can cause severe field and life problems. The present method does not lend itself well to an efficient assembly line operation and predrilling of bases and soldering pins on are time consuming and expensive. Since soldering discolors a brass base, nickel plated bases must be used for good cosmetics, therefore nickel plating is an added expense.
Since production operations are constantly changing, such as utilizing a threaded glass seal in place of dimples and an inner shell for attaching the base, the present method as described is not adaptable for use with a threaded mold lamp seal. Therefore, a need exists for a method of position orienting a metal halide lamp base assembly with respect to the curved arc tube.